Why do 250 mph (400 km/h) supercars have 0.40+ Cds?
 

Bugatti Veyron Cd 0.41 normal 0.36 lowered and preventing rear spoiler from deploying
Bugatti Veyron - Wikipedia, the free encyclopedia

Hennessey Venom GT 0.43 rear wing raised, 0.36 rear wing down
First Look at the New Hennessey Venom GT - Photos and Just-Released Details - Road & Track

Koenigsegg One:1 0.45-0.50
Koenigsegg One:1

They don't make THAT much downforce. At least not compared to all out racecars.
Are high Cd figures from cooling drag?

downforce to keep them on the ground.

regards
Mech

This is why:

https://www.youtube.com/watch?v=SFN_Gp1eHN0

-soD

Another example

https://www.youtube.com/watch?v=UJIK_e2a5Vs#t=35

It can't help. The Veyron has seven radiators and intercoolers.

I don't think it's downforce in all cases, the Koenigsegg CCX doesn't have much downforce, and neither does the McLaren F1 I think, though they don't hit 250mph.

The particular cars you listed have a lot of cooling drag thanks to the giant turbo motors.

Also keep in mind, fat tires. A 911 turbo S has Cd 0.31 claimed, even though it has only a tiny spoiler and an otherwise well-streamlined body, but it has huge 305 tires.

Considering any type of active downforce... those aren't bad numbers.

A winged racing car has typically two to three times as much drag as that... but it'll be producing a whole lot more downforce.

I would disagree with cooling drag, and I would say it's primarily downforce. These cars aren't designed for normal use.

Downforce is not just about keeping a car on the ground... Nor is it all about reducing the force of wind resistance. It's about being able to transfer 500-800 horsepower to the ground.

Nascar has a CD of .4, Indy car's are even worse, and DTM race cars are worse yet. Super cars are often designed in a similar fashion, because their needs are similar.

"To be faster you need power, but there is a limit to how much power you can put on the ground. To increase this limit, force to ground must be applied on the wheels. Increasing weight can do this, but weight makes handling worse and require more power. So we need some virtual weight, we call it downforce and get it from airflow around the car."

Speed and handling require downforce and not a super low CD. Allowing for more power to the ground, overcomes a "poor" CD. At 200mph, i think a 1 ft square creates drag that takes 70 horsepower to overcome. This is why they have 500-800 horsepower. Brute force combined with downforce allow them to be "super cars".

"In current motor racing competitions, including Formula 1, DTM, Indy cars and Touring Car, aerodynamic downforce plays the most important role in the performance of the cars."

Quoted From:
Downforce

Hope this helps ya.

~C

In addition, there's the matter of styling. Their bodies are shaped to fit people's misconceived ideas of what a fast car ought to look like, rather than what really is aerodynamically effective.

why
 

At these velocities,directional stability and safety are paramount to the manufacturers.They want their customers to live to buy another car.
They may do detail optimization to trim drag as long as it does not impact stability/safety.
Plan-views of these cars reveal drag-unfriendly profiles.

I believe that we have discussed this on here, although I did not see it in a search.

Most Efficient Racecar | Popular Science

This is the video of the Toyota hybrid knocking the Deltawing off of the track, but supposedly, since the Deltawing was smaller than the rest of the car, the Toyota driver could not see it, about 32 seconds in:
https://www.youtube.com/watch?v=FJAiZVuC3YI#t=32

Aerodynamic balance is real but so is cooling. When a car has about 1 MW engine power and 400 km/h top speed, it has to be able to dissipate huge amount of heat in various places. For that a supercar needs a lot air running thru cooling system. It's easy to see the effect of power. Let's check Cd-values of Mercedes CLA - one of the most aerodynamic cars available.
0,22 CLA 180 BlueEFFICIENCY Edition 90 kW (skinny tires)
0,25 CLA 180 90 kW
0,29 CLA 250 4MATIC 155 kW
0,32 CLA 45 AMG 4MATIC 265 kW

How high would the coefficient factor be if that chassis had 1 MW engine? I bet it would more that 0,4. Cooling for engine, cooling for transmission, cooling of brakes and huge tires increase drag a lot. A supercar has to be also very wide and relatively short which doesn't help.

Were those numbers reached with by placing the different engines in the same vehicle? Did each vehicle have the same trim level and body styling package? The answer is probably no.

skinny tires, low A-pillar shoulder with adapted A-pillar geometry, aerodynamically optimized exterior mirror housings and rear shape, optimized diffuser, optimized underbody and rear axle paneling, radiator shutter, aero wheel trims and serrated wheel spoilers on the front and rear wheel arches.


Different rear diffuser, rear fascia with side air outlet openings, Large side cooling intakes with flics (functional winglets), front splitter, different air dam height, different wheels, different suspension components, different exhaust, wider/larger tires, different underbody due to AWD system (underbody is an assumption, I can't find pics).

Details on the cars taken from Wiki and the MB website.

In regards to aerodynamics, these cars aren't the "same" vehicle... To display a difference in CD and say it's due to cooling, would seem to be over simplifying to me.


~C

Good point.
I recall that about 40% of fuel power goes wasted as radiator hot air, more than the power put to the ground so these cars vent out more than 1000 HP worth of hot air.
Meredith effect radiators are a must then.

It is more like 75% of the energy wasted as heat out of the radiator.

The Veyron actually has 10 radiators.

And the Koenigseggs all push past 250 with the CCX and after. I don't know much about the models before, I got into them around 2006. K labels the CCX as 245+, and the Agrera at 270+.

??? Cooling losses are not even close to the biggest proportion of losses in the typical engine.

Even if you count the extra power needed to overcome the extra drag due to the cooling requirements you will not get 75%.

I have a different opinion.Traction is a problem in relatively low speeds. Cars generate downforce at high speeds. Useful downforce is usually generated somewhere over the 200 km/h limit. For me downforce is all about cornering speed and high speed balance.

Those numbers were taken from Mercedes website (german). Most likely they are for basic model without any extra trim or spoilers. AMG-model has a bit different shape for outer decorstion for sure, but that doesn't explain a difference that huge.

I didn't say that it's only cooling. Wide tyres have also a major effect. And of cource there are many other thing but those two are always major reasons for high Cd values.

Whatever the reason, 400mph (644kph) cars *don't*.

I was trying to illustrate that each of those cars has a "base model" that is different. Some of them, seem to be far different from one another in terms of aerodynamics and the generation of down force. I mean one of them has radiator blocks to increase aero, but I could be entirely wrong :D . And simple exterior differences can make a large imapct on CD... Sometimes an increase and sometimes a decrease. Generating down force is not the same as reducing drag for FE. I don't claim to know what causes the difference in CD on these Mercedes, and you may be entirely correct, but here are some examples of how "little" differences make a big impact one way or the other. This is on the front of the car.

Data from modeling...
1. Stock 1990-1997 Mazda Miata
2. Stock 1990-1997 Mazda Miata at a 4in Ride Height
3. Small Front Air Dam at 4in Ride Height
4. Small Air Dam with Splitter at 4 in Ride Height
5. Large Air Dam at 4in Ride Height
6. Large Air Dam with Splitter at 4in Ride Height
Note: The air dam and/or splitter is 2 inches off the ground in study 3-6
http://i1291.photobucket.com/albums/...ps0ebf2c6a.png

The models illustrate how "simple" changes can drastically impact down force and drag at 100 mph... Much less 200+ mph. Creation of down force can cause a lower OR a higher CD value depending on how it is done. Taken from Splitter or Air Dam – Which Design is Best? | Hancha Blog.

Similarly, rear diffusers that are designed for down force, often have the opposite impact of those designed for better FE via lower CD. Most designs increase drag exponentially as the angle of the diffuser increases (gets steeper), and the angle is often quite steep in order to create more down force.
http://i.imgur.com/V9vou7Fl.jpg


Read the following description, the diffuser only created 55lbs of downforce @175mph with an increase of .002 in CD... How much would the CD increase if it was to generate 200 or 300lbs of down force on the same car? A modern F1 race care produces over 1000lbs of down force via the rear diffuser alone, just as an example of how race cars and super cars are designed differently from normal cars.

"The Werks1 diffuser was designed using the most state of the art engineering and computer modeling techniques available today. Using these processes, a functional prototype was created and wind tunnel tested in order to verify the effectiveness of the part. During testing, it was proven that a stock 997.2 (with wing raised) produced over 40lbs of lift at the rear axel, at 175mph. With the rear wing raised and the new Werks1 diffuser installed, that number was completely eliminated, and the test vehicle not only showed no signs of lift, but it now produced 14 lbs of usable downforce, again at the rear axle. When measured at both the front and rear axles, the stock 997.2 showed nearly 110 lbs of total lift. Once the Werks1 diffuser was added, total lift was reduced to just below 75lbs. Best of all, the addition of the Werks1 rear diffuser increased the vehicle's coefficient of drag (or Cd) by only .002, from .313Cd to .315Cd. What this means is that the downforce created by the Werks1 rear diffuser is essentially "free", with no negative side-effects to be found."
From Werks1 - Porsche 997.2 Carbon Fiber Rear Diffuser


Sorry if I implied that you thought it was only cooling, I was just trying to illustrate that little aerodynamic differences can create big differences to CD and the generation of down force at high speeds. Whether that is the case with the CLA, is unknown to me... But I feel like it would make sense :confused:




But do they turn at 400mph??? ;)

~C

Many models are similar in that sense and it doesn't make any sense at all to offer inferior aero parts in more expensive car. I doubt that Mercedes has engineered downforce in those models at least if we don't count AMG model. This increase of Cd value hand in hand can be seen in many other cars. It is actually the Newton's forgotten law.

Inferior? I don't believe that any model has "inferior" aero parts. They are just aero parts which have different goals. Aero parts designed for FE aren't inferior to those designed for downforce/venting. Just as those designed for down force/venting aren't inferior to ones designed for FE.

Why would we not include the AMG? You listed it as an example, it should be included. If we don't include the AMG then we shouldn't include the BlueEFFICIENCY Edition since it has changes made specifically for aero like the AMG does. IF we do that, we are only looking at a .04 increase in CD between the CLA 180 FWD (90kw) and the CLA250 4Matic AWD (155kw) models. And from what I gather, the underbody is different due to having a drive shaft and driven rear wheels which would impact drag compared to the FWD version... Of course I can't find comparison pictures for the two underbodies to see if this is true. I also assume AWD would lead to more heat/venting/drag than the FWD version.. Another factor that isn't taken into consideration when looking at CD relative to engine output.

And I agree, it can be seen hand in hand in other cars... Just as ground effects, diffuser, exhaust, underbody, airdam differences can be seen in many of those cars. I understand the science and theories behind heat exchange and drag... I don't deny that it exists. But I'm a hands on kind of person, lets actually look at some differences in models.


CLA 180:
Front end specific FE based aero designs, wheels with lower drag
http://i.imgur.com/CK1xbWFl.jpg


AMG 4MATIC
Massive air intakes with functional winglets, splitter, and non optimized front apron, drag creating 18" 5 spoke wheels
http://i.imgur.com/lyw1rb7l.jpg

Again, not saying it isn't heat exchange and "tires" that cause the difference in CD... I just find it hard to believe that those are the biggest causes when I look at the differences in design on these particular vehicles side by side. Especially after reading the projects on this site where people have done simple tweaks to decrease CD for better FE. Tweaks that are the opposite of what Mercedes did on the AMG (bigger intake/vents and larger open tires for example).


Sorry to the original poster for kind of high jacking this thread... I still stand by the generation of down force for application of power and safety with handling at speed :o:D

~C

I still think tires is the biggest difference. Porsche uses the exact same bodies across a lot of models. Between the Cayman S and non-S for example, the drag is the same, but the S is over an inch lower. Lowering 1 inch takes away as much drag as adding 1 inch of tire (mainly in the back). I think the number cited for each of these changes is about 0.01 Cd, which is quite a lot (3% of the whole car's drag).

Same story with the 911, you can see the 4S models with their fat tires having a couple hundredths of Cd added, but it's only an inch or so of tire width making that difference.

For the MB CLA, we're talking a huge change in tire width from the BlueEfficiency up to the AMG, I could see that alone explaining half of the increase in drag.

I couldn't find a general chart for tire width vs drag... But here is an example of increased drag from width.

CD --- tire & rim size (for a 1991 BMW 318i)

0.293 --- 155 R 15; 5 1/2 Jx15 St. with wheel covers
0.294 --- 165 R 15; 61 2 Jx15 St. with wheel covers
0.297 --- 175/70 R 15; 6 Jx15 St. with wheel covers
0.305 --- 185/65 R 15; 61 Jx15 St. with wheel covers
0.311 --- 205/60 R 15; 61 2 Jx15 St. with wheel covers
0.314 --- 205/60 R 15; 7 Jx15 LM
0.319 --- 225/55 R 15; 7 Jx15 LM
(http://ecomodder.com/forum/showthrea...g-cd-7475.html

Going from 6.1" (155) wide to 7.2" (185) increased by .012 CD, to 8.07" (205) is an additional .006 increase in CD... So if we use this as a benchmark just for conversation, a 1" increase will get an average of +/- .009 increase in CD. That's right in line with the .01 you cited above.

From Mercedes website, and the MB forums:
180 Blue Efficiency, stock 195/16 205/16 depending on country (7.6/8.07")
AMG model, stock 235/18 (9.25")

That's only 1.65" difference in tire size? So by your examples and the above chart, tires should/could account for less than .02 or .03 increase in CD :confused: I understand that percentage wise, that accounts for a lot of the cars drag, but it's still less than a third of the increase in CD from .22 to .32.



~C

Vents are there for cooling. More power needs more cooling. I really don't believe that downforce is that important in these machines. I don't have measurement data for CLA, but it's sister model A-Klasse was tested in SportAuto 3/2014. Even with optional AMG Aerodynamics package (AMG rear aerofoil, additional flics in front apron, larger front splitter) it didn't generate any downforce. It generated 430 N lift at the front end and 150 N lift at 200 km/h. Looks are there for sure but downforce is missing totally.

AMG model is the only model which has any change to be decigned downforce in mind.

Ok. Let's take an example, which doesn't have any aero packages or anything to do with downforce. Here are Cd values for Skoda Octavia:
1,6 l TDI/66 kW 0,286
1,6 l TDI/77 kW 0,288
2,0 l TDI/110 kW 0,297

1,2 l TSI/63 kW 0,294
1,2 l TSI/77 kW 0,290
1,4 l TSI/103 kW 0,301
1,8 l TSI/132 kW 0,304

If I understand correctly, all of these values are listed for standard base model with 195/65 R15 tires. If the tire size wuold increase, we would see a bigger change in Cd.

Okay now that you actually pull numbers up I'm starting to doubt myself.

I suppose the lack of wheel covers adds a little bit more, and the AMG probably has some kind of brake cooling duct which adds a little bit more. Still, does a radiator block really do that much? It appears to be the only difference. I am pretty sure all CLAs have underbody paneling.

If anything, I'm very skeptical about the 0.22 number. The Lexus LS460 has a longer tail and all sorts of aero features and fails to get close to 0.22.

By the way guys, I'm really enjoying this conversation! I've actually learned a good deal of things looking up the stuff I have posted! I hope you two (NHB, serialk11r) know that!


I can't find an english version of that article, or a site that will translate correctly :(

But I found this about the A45 AMG, so I find the generation of lift you claim a little bit suspect:
"The car only comes with three options - black wheels ($490), stiffer suspension ($1990) or the AMG aero package which adds front winglets, front wing and a fully sick rear wing which sets it further apart from the A250 Sport but also adds 40kg extra downforce at 250km/h"
(2013 Mercedes Benz AMG A45 | Top Gear)



I Understand that, but it's not the only one with modifications for aerodynamics.

Awesome, this is great! I could totally believe that +44kw would lead to .011+ in CD. I could also believe that +69kw would lead to .01+ in CD. Like I said, I understand and believe in the idea...

But take those ratios and apply them to the +175kw of the CLA:

Average the +44 and the +69, use +56.5kw gets us .01+CD.

+175kw = .034+CD

I can totally believe this figure! This would be around 1/3 of the increase in CD for the CLA. But this number would not be the "majority" of the increase in CD.

Lets say the tires account for .025 and the KW increase accounts for .034, that's .059. What causes the other .049 increase in CD :confused: Even if the two combined account for .06 of the .1 increase on the CLA, where does the other .04 come from? It must be aerodynamics of some type right?


~C

I'm skeptical about cooling causing so much drag. Twice the power does not mean twice the cooling drag necessarily. A bigger radiator would give you a lot more cooling capacity for the same air flow. You would think that with a bigger engine and a bigger turbo that they would add a bigger radiator as well.

I still say, the 0.22 is BS, they probably measured it without a "treadmill" which reduces drag from the wheels and underbody. Other car companies have wind tunnels too, you'd think at least *some* car some other company comes up with would have great aerodynamics, especially considering how there are coupes with good plan taper out there vs. the CLA sedan. If you believe the 0.22, then MB understands some aero black magic that no one else does, but then as you can see the CLA45AMG isn't doing so well at 0.32, so there's a contradiction somewhere.

Also a little more on the original topic, the McLaren P1 has a 0.34? Cd, and that's with fat tires + cooling ducts everywhere, and the wing has that bend in it that can't be good for drag when in the down position. It's got much better downforce and stability at speed than a lot of the other supercars, so maybe this isn't so much a design goal problem as it is a problem with poor design.

I've seen a wind tunnel vid on here where a tech trails smoke into the lower side grills of the CLA 180 blufficiency, and it exits through the front wheel wells. And all the other stuff they do, the protruding A pillars with precisely curved sides, all way beyond the usual stuff on other makes.

Yes I do believe the Cd is accurate.
And I think even the AMG does surprisingly well, only .04 worse than the Cd on my car.

The CD is different depending on whether it is in race or road settings... The ride height drops 50mm, the wing adjusts, and active aero in the front of the car adjusts as well. But I've never seen any data on how it imapcts CD... They say that the rear wing can reduce drag by 23 percent, but is that 23 percent bringing it lower than .34? Or is that 23 percent bringing drag back closer to .34 when in race mode?

I've also read, that they created active aerodynamic features to alleviate strain on suspension componets from the extreme downforce the car is capable of. Now THAT is crazy to me :eek: I think it makes like 1300 lbs of down force (the most of any production vehicle), so having that in mind... I think a .34 CD is simply amazing. And its probably due to the compact and super light body with inlet and vents wind tunnel tested/designed for optimal aerodynamic performance, especially in regards to interaction with the rear wing. Good read right here if you're interested: McLAREN P1 : McLaren Automotive Media Centre



~C

For what you need translation? It's quite simple data. I'm sure, that google translator can translate few simple phrases.

Top Gear is just car related entertainment. Their extra downforce can easily mean lift reduction. In my opinion everything here is just as it should be. 400 N @ 250 km/h means about 200 N @ 200 km/h. So the lift at rear end would be about 350 N @ 200 km/h without the spoiler. That sounds totally believable. SportAuto measured 380 N lift for Renault Megane RS.

Just take a look at those pictures you posted. What do you see at the front end of CLA? There are huge holes for brake cooling and probably for charge air too.

These modern cars don't have that much free space anywhere. At the front end there are already multiple radiators. And it's not only the engine cooling. 45 AMG engine uses very high boost pressure, which needs big radiators for water cooling. A high power four wheel drive automatic gearbox most likely needs healthy size radiator, but low power manual transmission doesn't need any extra cooling. We also have brakes, which have totally different specs in speed oriented AMG model compared to eco racers.

Mercedes Benz CLA 2013 Wind Tunnel Aerodynamic Test Part 2 - YouTube

It could if I could find the actual article...

Would the reduction of lift not come from the creation of negative lift? Is negative lift not "downforce"?

"Typically the term "lift" is used when talking about any kind of aerodynamically induced force acting on a surface. This is then given an indicator, either "positive lift" (up) or "negative lift" (down) as to its direction. In aerodynamics of ground racing (cars, bikes, etc.) the term "lift" is generally avoided as its meaning is almost always implied as positive, i.e., lifting the vehicle off the track. The term "downforce", therefore, should always be implied as negative force, i.e., pushing the vehicle to the road."
(Downforce)

Knowing the lift generated with the AMG aero package does us no good without knowing what the amount of lift measured *before* the aero package was added was. If lift was higher before the aero package, then the package created negative lift (downforce). Right? That's why I wanted to read the article to see if they listed the lift prior to the addition of the aero package.


You're right! The front end most likely creates more drag... I have never said that it doesn't.

I don't mean increase the cross sectional area, you can increase the thickness of the radiator, that's how it's usually done.

But compared to mainstream sports cars, let alone family cars, they do make a lot of downforce. A 2008-2010 Gen 4 Viper, for example, made a claimed 100 lbs downforce at 150mph, while the same car with ACR package (splitter, canards, and rear wing) made a claimed 1000 lbs at the same speed. It also lost 18mph on the top end, 202 mph for the regular coupe vs. 184 mph for the ACR.

The Veyron makes a claimed 733 lbs downforce in handling mode (up to 233 mph). The Venom GT makes a claimed 1050 lbs at 150 mph.

I think this gets at the real issue behind the higher drag numbers for exotic cars. Take a look at the plan views of various low-drag cars and their hypercar counterparts (especially the back halves):

Tesla Model S - Cd .24
http://blog.microbattery.com/wp-cont...3/02/pic-8.jpg

Gen I Honda Insight - Cd .25
http://www.insightcentral.net/_images/topdims.jpg

Gen III Toyota Prius - Cd .25
http://images.dealer.com/evox/stills...st0640_117.jpg

Chevrolet Volt - Cd .28
http://4.bp.blogspot.com/_FoXyvaPSnV...-Ampera-16.jpg

2014 Hyundai Sonata - Cd .25
http://cdn.zeegnition.com/sites/defa...?itok=OOZ7f9A_

VW XL1 - Cd .19
http://www.blogcdn.com/www.autoblog....swagen-xl1.jpg

Bugatti Veyron - Cd .41/.36
http://1.bp.blogspot.com/_V-MgyWMezl...-with-roof.jpg

Hennessey Venom GT - Cd .43
http://cs1.gamemodding.net/posts/201...0-56-23-71.jpg

Koenigsegg Agera - Cd .37/.33
http://image.motortrend.com/f/blogs/...1-top-view.jpg

Ferrari Laferrari - Cd .31
http://drive-a-ferrari.com/wp-conten...i-Top-View.jpg

McLaren P1 - Cd .34
https://s2.yimg.com/bt/api/res/1.2/k...larenp1top.jpg

It's not so much that they need that much downforce at 250 mph. They want usable downforce at 100 mph and they happen to have enough hp to go 250. The faster you go, the lower the Lift coefficient you need. ie. 100 lb will keep the car on the ground regardless of speed.

The Colani Corvette has a Cd of 0.19, and went 254 mph with 770 bhp. The whole car is an inverted wing.

In many cases, hennessey in particular, they're not designers or aerodynamicists. They slap together a bunch of available parts. He copies existing designs and slaps on twin turbos and a wing.

I don't think that explains it, at least not all of it. Almost all of the supercars have MORE plan taper at the greenhouse than the low-drag cars you mentioned. They also have side ducting that vents into the wake, which gets rid of some of the drag that comes from the wider rear.

The Veyron's rear has no taper to speak of, so that's probably bad. As someone else mentioned, the Hennessey Venom is just some Elise parts made a little bigger and slapped together, no surprise there (speaking of Lotus, the Exige comes from the factory with a ton of drag and not much downforce too). I also doubt that Koenigsegg does much careful aero optimization.

The areas I see on supercars that more drag comes from are:
1. Inlet scoops (fairly common, but are mostly pretty small and I don't imagine they add too much drag)
2. More radiators, other cooling ducts
3. Fat tires
4. Wings, canards, and other aero add-ons
5. Subtle things like the diffuser being angled higher to provide more downforce and more drag.

Without these things, I think the basic supercar shape (nearly perfectly streamlined greenhouse + nose) probably actually does very well, and the MP4-12C setup of having side scoops feed radiators that vent to the wake is probably a lot better than having radiators in the front.

All the aformentioned supercars have the engine (weight) in the back, and need large rear tires to balance the handling, not to mention to get traction at their high hp. That requires the "wrong" aero shape, but perfect aero takes a back seat to acceleration, braking and handling optimization. These cars aren't only about top speed, and they sure aren't about fuel economy. It is all a compromise for max performance. The 400mph land speed cars only need to do one thing, max top speed. They don't handle, don't brake, don't launch from a standing start.

Sure, the lack of plan taper doesn't explain all of it, but I would bet it's a significant factor. A perfect greenhouse taper won't do much for the car if the majority of the body terminates in a large wake area, which all these hypercars do. The departing angles are radically different on the lower-drag cars than the higher-drag cars. For the extreme eample, cf: XL1, which has as much greenhouse taper as the McLaren or Ferrari but a lower body taper to match which the latter cars lack. The trade-off is, it also has significantly narrower rear track, which would make a mid-engined RWD supercar so tail happy it would be nearly uncontrollable.

I guess my argument is that the basic shape has to be right or the car will never achieve low drag. The CLA is a good example of this: with the extra cooling inlets, wider tires, etc. it has a much higher drag coefficient even though the basic shape hasn't changed, but without the good shape it would not be able to achieve Cd .22 even with all the other details tuned in, and the AMG version drag would likely be even higher than it is.

Also, the Venom GT isn't just a bigger Exige body--it saw at least CFD development (there are images from some of those runs on the web), and I would suspect a wind tunnel at some point. With the active rear wing, that would be a necessity to get it to work correctly.